5. STRUCTURES AND DYNAMICS

The wide-field starcount programs provided new data on the
large-scale spatial structures of GCSs, and were
complemented later by CCD imaging with its ability to
reveal the inner GCS structures within the high background light of the
nuclear regions. The original paradigm that
globular clusters traced out much the same halo
structure as any other old Population II component
was broken by the discovery, in the Virgo giants M87 and M49,
that the GCSs are globally less centrally concentrated
than the halo light. On this basis, Harris & Racine (108)
and Racine (171) first suggested that the GCS might be older
than the bulk of the surrounding halo.

Not all galaxies, however, share this phenomenon. In the
Milky Way itself, the GCS seems to match the halo star
distribution fairly well (97), and to first order
the same agreement is found within other galaxies of both disk
and elliptical types (82,
87,
105,
106,
111).
Interestingly, two of these are the cD systems NGC 3311
(110) and NGC 1399
(105),
whose GCSs are structurally like those in M87
and M49 but whose halos are much more distended than
average. No case has been found in which
the GCS is more centrally concentrated than the halo.

If the projected GCS surface density is approximated by
a power-law curve cl ~ Rgc-, in giant ellipticals
is typically 1.5-2.0 for
Rgc 5 kpc (see
94 for
a comprehensive discussion).
Figure 6 shows two examples. For M87,
suggestions have often been made that its
large cluster population and extended
spatial distribution might have arisen from extra clusters
that were somehow added to its outer halo (see, e.g.,
56,
63,
221).
However, any such views are
incorrect. The M87 system is virtually identical in structure
with the prototypically normal M49 system at all
radii; that is, M87 contains three times more clusters than
normal at all locations, both inside and out. The high-SN
anomaly is thus a global characteristic, not easy to explain
by arguments special to one radial region (such as the high
mass in the extended X-ray halo of M87, or cluster formation in cooling
flows).

Figure 6. Spatial structure of GCSs in two giant Virgo
ellipticals, M87 and M49. The number of globular
clusters per unit area projected on the sky is plotted against
radial distance from the center of the galaxy. The filled
circles are from wide-field starcount data extending out to
r ~ 20' (see 94); the starred symbols are central
projected cluster densities from CCD imaging programs
(76,
103,
128).
Because the two sets of data have been shifted vertically for clarity,
the vertical scale is arbitrary.
For Virgo, 1 arcmin 5 kpc, so the profiles cover the
projected radial distance range from 2 to 100 kpc.
Even though M87 has a far larger cluster population,
the two GCSs are structurally quite similar at all radii.

Another quite general phenomenon in large galaxies seems to be
that cl(r) flattens off to a near-constant
level within Rgc 2 kpc. Observations right in to the nucleus of M87
(76,
114,
128)
show the effect, and similar conclusions have been reached
for several other systems including the Milky Way
(82,
103,
106,
172).
Dynamical destructive processes
(Section 7.1), which are
enormously more effective near the nucleus, may have virtually
removed whatever clusters were originally in these central
regions. In principal, these same processes might also
generate radial differences in the GCLF which should be
detectable. Observationally, little is yet known about any
such trends, though again in M87 (76,
128)
and the Milky Way (5), no obvious
variation of cluster luminosities with radius is seen.

In large disk galaxies, the GCS appears to be nearly spherical
in shape and, perhaps, rounder than the spheroid isophotes
[(82,
106), but see
H. Harris et al. (87) for a different comment on NGC 7814]. For giant
ellipticals, the GCS contours agree well with the ellipticity
and orientation of the galaxy isophotes
(38,
109,
117).
The smaller E6 system NGC 3377
has a GCS which is even more elongated than the
galaxy, perhaps as a residual effect of dynamical evolution
(106).

GCS kinematics and dynamics can be explored
through direct radial velocity
measurement of the clusters. Until recently
such data were available only for the Milky Way (e.g.
205)
and M31
(122,
and references cited).
However, velocities of sufficient precision (± 50 km s-1)
have now been obtained for some dozens of clusters in NGC 5128
(88,
116,
191), M87
(119,
151,
152),
and M49 (151).
These studies show that the velocity dispersions
are nearly constant with radius and
confirm the existence of dark-halo mass extending
outward with galactocentric distance as M(R) ~
Rgc to several tens of kiloparsecs. In
M87, the GCS velocity dispersion profile thus forms
a smooth
dynamical connection between the central galaxy light
and the still more extended X-ray halo.
Mould et al. (151) also find that the dispersion (vr) is
clearly higher for the GCS than for the underlying halo light
at the same location, and they argue that this is probably
a consequence of the larger spatial scale of the
GCS. Suitably anisotropic velocity distributions
are an alternate possibility.